A generally used resin sealing type semiconductor device includes a semiconductor chip 21, a die pad 22, an inner lead 23, a gold wire 25 and a sealing resin 27 as shown in FIG. 6. The semiconductor device is composed of a lead frame having a structure called "down set" in which the die pad 22 is formed in a lower level than the inner lead 23.
The assembly process of the resin sealing type semiconductor device will be explained below. First, a single semiconductor chip 21 is die-bonded to one surface of the die pad 22 of the lead frame. Then, in the die-bonded lead frame, the semiconductor chip 21 and the inner lead 23 are electrically connected through the gold wire 25 by the wire bonding technique. Further, the semiconductor chip 21, the die pad 22 and the inner lead 23, etc., are formed in a package using the sealing resin 27, thereby completing the assembly process of the resin sealing type semiconductor device.
The current market demands electric apparatuses with improved performances and functions, which are of compact size and light weight by mounting many components on a single substrate, or mounting at least the same number of components on a smaller substrate. To address this demand, it is required to improve the mounting density of the LSI efficiently, and the technique for sealing plural semiconductor chips in a single resin sealing device has been developed.
For example, as shown in FIG. 7, plural semiconductor chips 21 may be sealed in one package by mounting plural semiconductor chips 21 on one surface of the lead frame. This method employs the wire bonding technique, etc., when electrically connecting the semiconductor chip 21 and the inner lead 23, and connecting the semiconductor chip 21 and the semiconductor chip 21 both through the gold wire 25. In this conventional method, plural semiconductor chips 21 are mounted two-dimensionally, thereby presenting a problem in terms of mounting density such that the package itself becomes larger in size.
The technique of solving the above-mentioned problem is disclosed, for example, by Japanese Laid-Open Utility Model Application No. 147360/1987 (Jitsukaisho 62-147360). As shown in FIG. 8, to both surfaces of the die pad 22 which is not down set, two semiconductor chips 21 are respectively die-bonded as the back surfaces thereof are bonding faces. Further, the semiconductor chip 21 thus die-bonded and the inner lead 23 are connected using the wire bonding technique. Thereafter, the semiconductor chip 21 and the inner lead 23 are formed in a package using the sealing resin 27.
Another semiconductor device is disclosed by, for example, Japanese Laid-Open Patent Application No. 105450/1990 (Tokukaihei 2-105450) having a structure shown in FIG. 9. The semiconductor device is assembled in the following manner. First, the semiconductor chip 21 is die-bonded to the lead frame which is not down set, and process up to the wire bonding process is performed. Then, two lead frames thus processed are stacked in such a manner that the respective back surfaces thereof are in contact with one another so as to seal the resin.
In order to achieve a thinner semiconductor package for mounting thereon semiconductor chips on both surfaces of the lead frame, it is required to adopt an extremely thin resin on the bonding wire. For example, in the structure shown in FIG. 8, in the case where the semiconductor package with a thickness of 1 mm is adopted, and the respective thicknesses of the die pad 22 and the semiconductor chip 21 are set to 0.1 mm and 0.2 mm, and the height of the gold wire 25 is set to 0.15 mm, the resin on the gold wire 25 on one surface becomes extremely thin (thickness of 0.1 mm).
In the conventional semiconductor package, the portion of the die pad is supported only by a support bar. Therefore, due to the pressure of the resin applied to the semiconductor chips in the molding process using the resin, the support bar for supporting the die pad is easily deformed, and the die pad is formed in a slanted position. In this case, as the sealing resin is thin, the gold wire and the semiconductor chip protrude from the surface of the package, thereby presenting the problem that the semiconductor chips become unpresentable.
In the package structure where the semiconductor chips are mounted on both surfaces of the lead frame, since the die pad is not down set, the die pad is easily deformed when forming the mold.
On the other hand, since the leading end of the inner lead is separated lead by lead, when adopting the thin lead frame, the inner lead may be deformed by the pressure from the resin applied in the molding process, thereby presenting the problem that the gold wire may be pushed up and protrude from the surface of the package, or the gold wire may break or contact the adjoining inner leads.
Alternatively, when sealing semiconductor chips of a large size with the resin, in the conventional package, since the die pad portion and the leading end portion of the inner lead are separated, it is necessary to ensure at least a predetermined distance between the end of the semiconductor chip and the outer side of the resin portion of the package. Moreover, in general, since only the leading end portions of the inner leads are silver-plated for electrically connecting to the semiconductor chips, the semiconductor chips of the large size cannot be sealed in a package of the same size as the compact semiconductor chips. Namely, it is necessary to set the distance from the semiconductor chip to the bonding position of the inner leads not less than the distance between the die pad and the leading end portions of the inner leads. Therefore, in the case of sealing the semiconductor chips of the large size with the resin, a large size package is required.
In order to die-bond the semiconductor chips to both surfaces of the lead frame, it is necessary to displace the bonding positions of the inner lead side when the wire bonding process is applied. This has been the drawback of sealing the semiconductor chips of the large size.
As described, in order to contain two semiconductor chips in a single semiconductor package, it is necessary to prevent the die pad, to which the semiconductor chips are die-bonded, from being inclined by the pressure of the resin applied in the molding process as surely as possible. Further, when sealing the semiconductor chips of the large size, it is necessary to set the distance between the semiconductor chips and the wire bonding positions shorter in order to prevent an increase in size of the package.